Tanks should be set up for at least 6 months before adding clams.
The 2" maxima is kinda still in the danger zone for maxima's. Maxima's are the most complicated to keep and 2" or less have a high mortality rate.
Youre lighting seems ok maybe a little much, do your clams seem to be bleaching? If so try moveing them down or adding some sheets of window screen above them to dim the light a little.
Your fish concern me, the tangs could very well be nipping at the mantle's. Angel's and wrasse's are known to be not safe with clams.
Your ph seems a little low and alk is kinda borderline high. bring your ph to 8.2
Pictures would help greatly, and also if you think it might be the fish bothering them you can cut the bottom off a 2 litre bottle, drill a bunch of 1/4" holes in it and wash it out good, then place the clam in the sand and put the bottle over the clam to isolate it from the fish.